Personal computers, word processors and video printers are widely used devices many of which utilize heat-sensitive transfer printing as a technique for obtaining a hard copy of a document.
A typical recording medium 10 for heat-sensitive transfer printing is shown in FIG. 1. The recording medium 10 includes a first substrate 20 such as a polyester film. An ink layer 30 is formed on one side of the first substrate 20. The ink layer 30 includes a dye such as a heat-sublimate dye or a heat-melt dye and a binder resin for holding the dye. The ink layer 30 is adhered to the first substrate 20 by an adhesive layer 40. A heat-resistant and lubricant layer 50 is formed on an opposite side of the first substrate 20. The heat-resistant and lubricant layer 50 serves to prevent or substantially reduce sticking of the recording medium 10 when the recording medium 10 is placed in contact with a thermal head (not shown) and to enable smooth running of the recording medium.
A typical method for heat-sensitive transfer printing on a second substrate (not shown) having a dye-acceptor layer is as follows. The ink layer 30 which includes a dye is placed in contact with the dye-acceptor of the second substrate. A thermal-head is then utilized which provides heat to the heat-resistant and lubricant layer 50 in accordance with predetermined image information. The heat provided by the thermal head then causes the dye from the ink layer 30 to be transferred to the dye-acceptor layer to form an image.
It is common to use polyurethane cross-linked by a hardening agent containing di-isocyanate as an adhesive composition utilized to form the adhesive layer 40. The reason is that polyurethane adhesive composition adheres to the polyester film used as a substrate.
However, the use of such a polyurethane adhesive composition has disadvantages. A disadvantage is that the polyurethane adhesive composition provides an insufficient adhesive force between the ink layer 30 and the first substrate 20 for heat-sensitive transfer printing. Consequently, the ink layer 30 peels off from the first substrate 20 during the heat-sensitive transfer printing process and sticks to the dye-acceptor layer of the second substrate. Another disadvantage is that the polyurethane adhesive composition has a relatively short pot life (about 12 hours) which hinders the manufacturing process and is not convenient to handle.